Many electronic devices include a user interface device for allowing user interaction and user input. One user interface device is a button or a key. Conventional buttons include mechanical components to actuate a switch to indicate a button press or button activation. Mechanical buttons also provide a tactile feedback to the user to indicate the button has been pressed. More recently, touch-sensor buttons are being used in some applications to replace mechanical buttons. One type of touch-sensor button operates by way of capacitance sensing, utilizing capacitance sensor elements. The capacitance detected by a capacitance sensor changes as a function of the proximity of a conductive object on the sensor element. The conductive object can be, for example, a stylus or a user's finger. In a touch-sensor button, a change in capacitance detected by each sensor due to the proximity of a conductive object can be measured by a variety of methods. Regardless of the method, usually an electrical signal representative of the capacitance detected by each capacitance sensor is processed by a processing device, which in turn produces electrical or optical signals representative of the button or sensor activation of the touch-sensor button.
FIG. 1A illustrates a conventional sensing device having three touch-sensor buttons. Conventional sensing device 120 includes button 121, button 122, and button 123. These buttons may be capacitive touch-sensor buttons. These three buttons may be used for user input using a conductive object, such as a finger.
In general, capacitance touch-sensors are intended to replace mechanical buttons, knobs, and other similar mechanical user interface controls. However, one disadvantage of capacitance touch-sensors over mechanical buttons is that zero force may be required to activate the sensors, resulting in higher possibility of unintentional activations of the sensors. Using capacitance touch-sensor buttons, especially in household devices, may be unintentionally activated by a presence of water or other conductive liquids, such as grease, wastewater, sludge, fruit juices, yoghurt, milk, alcohol, acids, caustics, water-based emulsions, pulp and paper slurries or the like, on a touch panel of the touch-sensor buttons. Capacitance touch-sensor buttons are configured to detect the capacitance variation introduced by a user's finger (or other conductive objects). However, a water drop on the button may act as a bridging conductor that may unintentionally activate the touch-sensor button. The unintentional activation of touch-sensor button presents a serious safety issue for some household appliance, especially ovens, stoves, microwaves, blenders, heaters, or the like.
In one conventional design, a touch-sensor button, as illustrated in FIG. 1B, includes a capacitive touch-sensor button 130. Capacitive touch-sensor button includes two or more conductors, such as conductors 135 (e.g., grounded conductors) and sensor element 136. When a conductive object, such as finger 133, is placed in proximity to the sensor element 136, there is a capacitance, Cf, between the sensor element 136 and the conductive object with respect to a common ground 137. In a touch-sensor button design, typically one conductor, sensor element 136, is sensed and the sensor element 136 is surrounded by a fixed ground. The fixed ground may be one or more conductors 135 that are connected to system ground 138. There is also parasitic capacitance Cp between the conductors 135 and sensor element 136. The capacitance between the electrodes when no conductive object is present is the base capacitance Cp that may be stored as a baseline value. There is also a total capacitance (Cp+Cf) on the sensor element 136 when the conductive object (e.g. finger 133) is present on or in close proximity to the touch-sensor button 130. The baseline capacitance value Cp may be subtracted from the total capacitance when the conductive object is present to determine the change in capacitance (e.g., capacitance variation Cf) when the conductive object is present and when the conductive object is not present on the sensor element 136. Effectively, the capacitance variation Cf can be measured to determine whether a conductive object is present or not (e.g., sensor activation) on the touch-sensor button 130. The capacitance on the sensor element 136 may be measured using conventional capacitance sensing techniques, such as using a relaxation oscillator circuit or a charge transfer circuit.
In this conventional design, the sensor element 136 is surrounded by a fixed ground of conductors 135, which are connected to system ground 138. The conductors 135 are disposed in the same plane as the sensor element 136. The conductors 135 are disposed in the same plan as the sensor element 136 to achieve larger capacitance variations (Cf) on the sensor element 136 when the finger is touching or in close proximity to the touch-sensor button 130, as compared to a touch-sensor button that is not surrounded by a grounded conductor. Surrounding the sensor element 136 with grounded conductors may increase the possibility that the water 134 (e.g., water drop or film of water) on the overlay of dielectric material 134 of the touch-sensor button acts as a bridging conductor. The capacitance introduced by the bridging conductor between the conductors 135 and sensor element 136 and the water 134 may be sufficient to unintentionally activate the touch-sensor button 130. In other words, the capacitance introduced by the bridging conductor is in the same range as the capacitance Cf as measured to detect the presence of a conductive object, for example, finger 133. If the capacitance introduced by the bridging conductor is greater than a presence threshold, button activation occurs unintentionally.
As previously described, the unintentional activation of touch-sensor button presents a serious safety issue. Many household appliances, as well as other devices, are commonly exposed to such elements as water or humidity, which may unintentionally activate the touch-sensor button of the appliance. Similarly, in many industrial appliances, the touch-sensor buttons may be exposed to other conductive liquids, which may unintentionally activate the touch-sensor button of the appliance. Unintentionally activating the touch-sensor button on some household appliances, especially ovens, stoves, microwaves, blenders, or heaters, may present serious dangers to the appliance itself, the consumer, and/or the consumer's property. Serious dangers may also be presented by unintentional activation of touch-sensor buttons on some industrial appliances.